Animation is a process by which a series or sequence of hand or computer drawn images are combined in such a way as to appear to the viewer that an image or multiple images have moved from one frame to another. As films and videos are actually a sequence of still pictures linked together, it is necessary to relate the images from one frame to another so that the transition appears fluid when shown at the speed of the final product (30 frames per second for video).
The most prevalent animation technique is called cell animation. In cell animation, various layers or "cells" are hand drawn by animators on transparencies (or on paper which is later transferred to a transparency), depending on the stage of the process. Typically, each cell tracks an individual object. This type of a system is referred to as a "paper" system.
For a typical cell animation application, the first step is to draw the image in stick or outline form for the first frame. Next, this image is redrawn as it would appear a number of frames into the future. For purposes of example, this future frame is nine frames into the future. After these two frames are drawn, the "in-betweening" step takes place. Here, the "in-betweener" begins by drawing the frame which occurs in the middle of frames 1 and 9 (frame 5). After this step, the frame between the extreme frame and the first in-between frame (frames 1 and 5) is drawn, and so on until all of the images occurring in successive frames (1 to 9) have been drawn. In-betweening in paper systems is accomplished with backlit paper, so that the outer frames are in the in-betweener's view when he or she is drawing the in-between frames.
The first pass of "in-betweening," called pencil testing, is usually drawn in pencil or on individual pieces of paper that are pin registered. The paper drawings are then videotaped for viewing the accuracy of the animation. This allows for verification of lip synching, expression and movement of the elements of a set of frames. The next pass is called inking, where the pencil drawing is traced with clean lines drawn in ink. The third step involves photocopying, followed by hand painting, and the final step of compositing. In compositing, all of the cells (layers comprising each animated image) for an individual frame are stacked on top of one another and photographed to make up each frame for the animated sequence. Traditional cell animation uses approximately three layers (transparencies), where each layer is considered a "cell." Each cell is drawn in outline form, and then turned over and painted from the back. Next, each cell is layered, one on top of another, to produce a final image on film. In reality, though three layers are involved, there may actually be four or five cells produced. This is because each layer may itself involve multiple cells.
To produce high quality and more realistic animation, it is always essential to have the animated images track the sound or speaking with which the images will be displayed. To ensure this identity between sounds and images, the soundtrack and speaking parts are usually recorded prior to the animated images, and the animator, in creating the images, tracks the sound. This means that, as an animated character is speaking, the animator draws the mouth and facial expressions to sync with the pre-recorded soundtrack.
There are various software programs on the market which facilitate the drawing of images. CAD (Computer Animated Design) programs use an equation to generate a series of screen pixels between two points. A CAD program, then, may represent an image being drawn as a set of vectors. The use of an equation to represent the image information allows for complicated effects, such as image manipulation, translation and rotation.
Other drawing programs work with raster-represented image information. This is also referred to as bit mapping. In this technique, an image is drawn and stored as a map of pixels on the screen. Manipulation of images is much more limited, since there is a one-to-one correlation between what is stored for each pixel and what is displayed for each pixel. This is in contrast to an equation or a vector based system, where each pixel can be altered by changing the variables in the equation.
One benefit, however, of a raster-representation system is its simplicity, as contrasted with the complex calculations occurring in a vector or graphics-based system.